A major question about the structure and assembly of filimentous bacterial viruses is how the DNA helices and the coal proteins interact with each other. Comparative studies of many different viruses systems are involved. - Mass mapping by the STEM is an integral part of this. Determinations of mass/length and length, radial and axial mass distributions, are needed. STEM3 will be used to determine the radial position of phosphorus atoms in the DNA. This will test the hypothesis that some strains have the phosphates near the axis and bases pointed outward.